Nonwoven fabrics or webs constitute all or part of numerous commercial products such as adult incontinence products, sanitary napkins, disposable diapers and hospital gowns. Nonwoven fabrics or webs have a physical structure of individual fibers, strands or threads which are interlaid, but not in a regular, identifiable manner as in a knitted or woven fabric. The fibers may be continuous or discontinuous, and are frequently produced from thermoplastic polymer or copolymer resins from the general classes of polyolefins, polyesters and polyamides, as well as numerous other polymers. Blends of polymers or conjugate multicomponent fibers may also be employed. Methods and apparatus for forming fibers and producing a nonwoven web from synthetic fibers are well known. Common techniques include meltblowing, spunbonding and carding.
Nonwoven fabrics may be used individually or in composite materials as in a spunbond/meltblown (SM) laminate or a three-layered spunbond/meltblown/spunbond (SMS) fabric. They may also be used in conjunction with films and may be bonded, embossed, treated or colored. Colors may be achieved by the addition of an appropriate pigment to the polymeric resin. In addition to pigments, other additives may be utilized to impart specific properties to a fabric, such as in the addition of a fire retardant to impart flame resistance or the use of inorganic particulate matter to improve porosity. Because they are made from polymer resins such as polyolefins, nonwoven fabrics are usually extremely hydrophobic. In order to make these materials wettable, surfactants can be added internally or externally. Furthermore, additives such as wood pulp or fluff can be incorporated into the web to provide increased absorbency and decreased web density. Such additives are well known in the art.
Qualities such as strength, softness, elasticity, absorbency, flexibility and breathability are readily controlled in making nonwoven fabrics. However, certain properties must often be balanced against others. An example would be an attempt to lower costs by decreasing fabric basis weight while maintaining reasonable strength. Nonwoven fabrics can be made to feel cloth-like or plastic-like as desired. The average basis weight of nonwoven fabrics for most applications is generally between 5 grams per square meter and 300 grams per square meter, depending on the desired end use of the material.
Nonwoven fabrics have been used in the manufacture of personal care products such as disposable infant diapers, children's training pants, feminine pads and incontinence garments. Nonwoven fabrics are particularly useful in the realm of such disposable absorbent products because it is possible to produce them with desirable cloth-like aesthetics at a low cost. Nonwoven personal care products have had wide consumer acceptance. The elastic properties of some nonwoven fabrics have allowed them to be used in form-fitting garments, and their flexibility enables the weaver to move in a normal, unrestricted manner. This combination of properties has also been utilized in materials designed for treating injuries. Kimberly-Clark's FLEXUS.TM. wrap, for example, is effective in providing support for injuries without causing discomfort or complete constriction. The SM and SMS laminate materials combine the qualities of strength, vapor permeability and barrier properties; such fabrics have proven ideal in the area of protective apparel. Sterilization wrap and surgical gowns made from such laminates are widely used because they are medically effective, comfortable and their cloth-like appearance familiarizes patients to a potentially alienating environment.
Various mechanisms have been employed for increasing the integrity of nonwoven webs such as spunbonded filament webs. Bonding of nonwoven webs can be accomplished by a variety of methods typically based on heat and/or pressure, such as through air bonding and thermal point bonding. Ultrasonic bonding, hydroentangling and stitchbonding may also be used. There exist numerous bonding and embossing patterns that can be selected for texture, physical properties and appearance.
One method is compaction, in which the web is passed between heated and/or unheated nip rollers to cause interfilament bonding. Another known mechanism is the hot air knife. A hot air knife is useful in bonding the individual polymer filaments together at various locations, so that the web has increased strength and structural integrity. Hot air knives are also used for aligning meltblown fibers during manufacture of meltblown webs, for cutting nonwoven fabrics, for chopping reclaim, and for a variety of other uses.
One use of the hot air knife is to improve the structural integrity of nonwoven webs before passing them through standard inter-filament bonding processes. Through-air bonding "TAB") is a process of bonding a nonwoven bicomponent fiber web in which air sufficiently hot to melt one of the polymers in the fibers of the web is forced through the web. The air velocity is between 100 and 500 feet per minute and the dwell time may be as long as 6 seconds. The melting and resolidification of the polymer provides the bonding. TAB has relatively restricted variability and since TAB requires the melting of at least one component to accomplish bonding, it is most effective when applied to webs with two components like conjugate fibers or those which include an adhesive. In one method, air having a temperature above the melting temperature of one component and below the melting temperature of another component is directed from a surrounding hood, through the web, and into a perforated roller supporting the web. Alternatively, the through-air bonder may be a flat arrangement wherein the air is directed vertically downward onto the web. The operating conditions of the two configurations are similar, the primary difference being the geometry of the web during bonding. The hot air melts the lower melting polymer component and thereby forms bonds between the filaments to integrate the web.
The TAB process requires the web to have some initial structural integrity, sufficient to hold the web together during TAB. The hot air knife has been used to provide nonwoven webs (e.g., spunbond webs) with initial structural integrity prior to TAB.
A conventional hot air knife includes a manifold with a slot that blows a jet of hot air onto the nonwoven web surface. U.S. Pat. No. 4,567,796, issued to Kloehn et al., discloses a hot air knife which follows a programmed path to cut out shapes needed for particular purposes, such as the leg holes in disposable diapers. U.S. Pat. No. 5,707,468, issued to Arnold et al., discloses using a hot air knife to increase the integrity of a spunbond web. U.S. patent application Ser. No. 08/877,377, to Marmon et al., filed Jun. 17, 1998, discloses a zoned hot air knife assembly used to heat discrete portions of a nonwoven web.
Hot air knives have proven useful in many areas. However, as explained above, they require large quantities of air heated to high temperatures, in order to be effective. There is a need or desire for techniques which improve the heating efficiency of hot air knives used to heat nonwoven webs, thereby lowering the energy requirements and associated costs.